R—Fe—B permanent magnets have been produced for many years by so-called dry molding methods, in which dry fine powder is molded in a die while applying a magnetic field. In the dry molding method the concentration of oxygen in a nitrogen or Ar gas, a pulverization medium, is usually controlled in a desired range by introducing a trace amount of oxygen into a jet mill in the fine pulverization of a coarse starting material powder in the jet mill. This is to cause the oxidization of fine powder surfaces. Finely pulverized powder would be burned without this oxidation treatment, when brought into contact with the air. The fine powder subjected to the oxidation treatment has an oxygen content of 5000-6000 ppm, and the sintered body obtained from this fine powder has an oxygen content of 4000-5000 ppm. Most of oxygen in the sintered body is bonded to rare earth elements such as Nd, etc., existing as oxides in the grain boundaries. To supplement an oxidized part of the rare earth elements, the total amount of rare earth elements in the sintered body should be increased, resulting in decrease in the saturation magnetic flux density of the sintered magnet.
To solve the problems of the dry molding method, JP 7-57914 A proposes a method for producing a sintering rare earth magnet comprising the steps of injecting a mixture of rare earth magnet powder and a mineral oil or a synthetic oil under pressure into a die cavity, to which an oriented magnetic field is applied, wet-molding it in a magnetic field in a low-oxygen atmosphere to form a ring-shaped green body, removing the solvent from the green body, and sintering the green body in vacuum. This method can stably produce high-performance, sintered R—Fe—B permanent magnets having a small total amount of rare earth elements and a small oxygen content. However, because the slurry is injected under pressure into the die cavity, to which the oriented magnetic field is applied, the fine R—Fe—B powder having large spontaneous magnetization oriented is subjected to large constraint by interaction with the oriented magnetic field, resulting in a nonuniform filling density in the die cavity. As a result, the resultant green body has a nonuniform density, causing deformation and cracking in the resultant sintered body. Also, because the slurry is injected into the die cavity under pressure toward a core center through an injection aperture open in the die cavity, the slurry impinging the core is divided to flows in two directions, which are converged on the opposite side of the injection aperture by 180°, so that the resultant sintered body has cracks generated from this converging position.
JP 11-214216 A proposes a method for producing a sintered R—Fe—B permanent magnet comprising the steps of ejecting a slurry of an R—Fe—B permanent magnet powder and a solvent such as a mineral oil, a synthetic oil or a vegetable oil through a slurry-supplying pipe inserted into a die cavity, to which a magnetic field is applied, molding the slurry filled in the cavity under pressure while gradually withdrawing the slurry-supplying pipe from the cavity, removing the solvent from the resultant ring-shaped green body, and sintering the green body. Because the slurry is injected into the die cavity through the slurry-supplying pipe inserted deep into the die cavity in this method, the die cavity is filled with the slurry at a good filling ratio even in the case of molding a relatively long ring-shaped green body. However, because the slurry-supplying pipe is inserted deep into the die cavity and withdrawn while ejecting the slurry, this method is disadvantageous in a long supplying time of the slurry. In addition, the slurry-supplying pipe leaves a void in the resultant green body at a position thereof, and this void acts as a starting position of cracking in the resultant sintered permanent magnet.
Proposed as another method for producing a radially anisotropic ring-shaped R—Fe—B permanent magnet is a method comprising the steps of pulverizing quenched ribbons of an R—Fe—B magnet alloy, molding the resultant powder at room temperature, hot-pressing the resultant green body in an inert gas atmosphere for densification, hot-plastic-working the resultant hot-pressed body to form a cup body provided with radial magnetic anisotropy, and cutting a bottom portion off to provide a ring-shaped product (JP 9-275004 A, JP 2001-181802 A). However, because the hot plastic working of the hot-pressed body in an inert gas atmosphere is carried out at a relatively low temperature of about 700-800° C. so that crystal grains do not grow too much, it should be conducted at an extremely low speed to prevent cracking. Though different depending on the size of a magnet, one hot plastic working operation usually takes 10-30 minutes, low productivity as an industrial method for producing permanent magnets. In addition, because pressed bodies thus produced are likely to have cracks in their end portions, cracked portions should be cut off. For these reasons, this production method suffers from a high production cost. Further, the resultant ring magnet has large variations of magnetic properties. Though the degree of radial anisotropy depends on how much deformed in the hot plastic working, particularly small-diameter products and long products having large hot plastic working resistance suffer from large variations of a surface magnetic flux density.